Again, I am reacting to the statement “I wouldn’t advise it for everybody. But when you reach the point where you can be completely raw and honest with someone without fear of being judged, it feels really good.”
But if the “raw and honest” or “brutal honesty” (your words) are critical about the other person, why can’t the other person be critical in return about the brutal honesty?
So, “You look fat in those pants” leads to “your ‘brutal honesty’ is an excuse to be an asshole.”
Instead of strictly content, look at the TIMING of the message. Discussions about dissatisfaction with sex (again, not “hey, I’m not crazy about that thing you do with your fingernails,” but rather “you’re just not doing it for me, and I’ve faked every orgasm…”) shouldn’t happen in the bedroom, and damn sure shouldn’t happen during post-coital cuddles. Discussions about your lady looking grossly overweight in that dress don’t happen in the middle of the wedding reception she’s wearing it to, but in the privacy of your home before you leave. Conversations about being a horrible driver don’t happen as you’re careening down the highway on the wrong side of the road.
There is nothing wrong with wanting honesty in a relationship; there’s something desperately wrong with not understanding that there’s a time and place when that honesty will be appreciated, heard, and implemented. “Hey, honey, the wedding reception is two months away, and I know you’ve been wanting to lose some weight do you can look great in your new dress. Want to start working out with me?” is perfectly acceptable and may get you the positive results you’re both looking for. Tapping on your glass and announcing to a room full of guests that your wife is a cow who couldn’t push away from the table is going to get you a divorce. And that’s the equivalent of what the OP is doing.
Time and place, people. It’s not difficult.
Ok, then. Let’s assume your butt is too big, your genitals smell bad, and you give bad oral.
If you’re unaware of it, would you want to know, or not? If you don’t want to know, we’re just so different that the debate is pointless. If you do want to know, then why would you lie to your partner about the same things?
If you’re aware of it (and presumably don’t want to or can’t do anything about it), will you ask your partner for instance “doesn’t my cock smell amazingly good?”. And if you do, again, we’re so different that a discussion is pointless, but at least I’d like to know why you’d be asking such a question in this case.
That’s actually a really, really good time for that conversation. 
Presumably because the other person appreciates it. I’m asking this again (probably the third or fourth time) : how can you trust what your partner says if he typically lies to you in order to spare your feelings? How can you go to the bottom of an issue if everybody dances around uncomfortable truths?
Niceties are fine with strangers, but you might want more from a partner. If you can’t be honest with him, with whom can you?
(and “brutal honesty” are in fact the OP’s words)
And explain to me again how I’m an asshole for answering truthfully to a question that you asked? Again, what’s the fucking point of this question? Why do you want to be lied to? Especially when you know you’re going to be lied to?
I don’t think my partner lies to spare my feelings. I also don’t think he would likely say that he indulges in brutal honesty and it’s great that he’s not judged for it.
There is a middle ground: having relationships with people about whom we can be mostly positive, and couching criticisms in ways that are not brutal. And when we fail at framing things the right way, accepting that the other person gets to judge our words and our motives just as we get to judge whatever it is about the other person we were speaking up about. We should all be judged. The partner should not just have to accept without complaint that we’re going to say asshole things.
See, but there’s a vast and important difference between tactfully - though honestly and forthrightly - informing your partner that she’s got an ass that’s big enough to have its own zip code and using the post-coital glow and cuddle moment to discuss her sexual shortcomings. The OP here - by his own admission - isn’t being honest with a long-time partner about her failings. He’s insulting a relatively new sexual partner about her physical attributes - and he’s doing so at a time that is a) inappropriate and b) has the maximum possible chance of being hurtful to her.
You are making a critical error in thinking, sir. “Tactful” and “dishonest” are not synonyms. If I look like shit, I fully expect my husband to let me know about that. I won’t even be mad at him about it. I will, in fact, probably be grateful. That is, if my husband has the native common sense to do so at a time and in a manner that is appropriate and relatively tactful. Like, say, letting me know that a dress I’m wearing makes me look like an overstuffed sausage before we leave the house, as opposed to making my unfortunate fashion choices a topic of dinner conversation at a party.
If your sexual partner is asking you about her performance or physical attributes just after sex, surely there must be SOMETHING positive you can say. Her need for reassurance at a vulnerable and open moment is totally understandable and valid - you don’t have to lie to her. If you don’t have something positive to say, why in the name of God are you having sex with her at all? Mr. OP is taking the opportunity offered by a new* sexual partner to say something reassuring and using it to make derogatory comments about his new sexual partner’s body. I am damn shocked the lady went out with him again at all, period, let alone long enough after such a moment that he felt compelled to inquire why he wasn’t getting any anymore.
The answer, OP, to why you weren’t getting any anymore, for the record, is “Because the last time we fucked, you insulted my body and got all butthurt because I didn’t “appreciate your honesty”.” You weren’t being honest, you were being an asshole.
*It is worth pointing out - again - that it’s not like the OP and his former flame were in a long term relationship. It was a new relationship. Total honesty is the sort of thing that people build up to in a stable relationship. Vanishingly rare is the soul who will react favorably to an introduction like “Hi, I’m Jesse. Your ass is kind of big and your chin’s a little weak and man have you thought about Clearasil for that acne, but you never know about people, so I guess I’ll give you a try and see if you’re worth dating. You can pick me up at 7 on Thursday.”
There’s a middle ground between brutal honesty and being lied to. If I do have an unpleasant smell in my genital area, I want to know, I don’t want to be lied to. If it was addressed as something like “honey, I don’t think you realize it, but you sometimes have a pretty bad smell that needs to be addressed. Maybe here are some different things you can try…” then I would be embarrassed, but grateful and receptive.
But if it was something like “I haven’t told you this until now, but you seriously stink. You have a stench that makes me want to gag sometimes. None of the women I’ve been with have smelled half as bad as you,” then I would be insulted and angry. I might address the issue, or I might think that if someone is such a jerk to tell me something like that in that way, then he might also be a big enough jerk to see a problem where one doesn’t exist.
I sort of feel like that’s more the time for incoherent screaming and possibly praying. 
I also feel like ** Clair** is being deliberately obtuse.
Sorry for the slight hijack, but I have to ask. Is this really a thing? Can a penis, or many penises, really stretch out a woman? I find that a bit implausible. Giving birth to a baby I could maybe imagine doing something, but even then, things do tighten up again don’t they? I am a bit doubtful this is at all correlated with loose morals.
You keep ignoring this fact: People talk to each other at different times for different reasons.
Sometimes people talk in order to gather information. If I’m trying on a pair of pants at a department store and I ask my wife to come in and tell me if it fits, and she says, “Well, it’s the right length, but it’s too tight around your posterior,” then that is an appropriate, honest response.
However, there are many other times–and depending on your relationship with that particular person, it might be the majority of times–when human interaction is not about an exchange of information. When you are in that context, an appropriately socialized adult human knows what is going on.
So, if in the afterglow, someone says “That was so hot, wasn’t it?” The purpose of the conversation is not an exchange of information. You are not being asked a factual question, so it is not lying if you do not respond with a factual answer.
If the purpose of the interaction is not to “get to the bottom of an issue,” then you’re being an asshole if you treat it as if it is.
Furthermore, most people know how big their asses are, and a lot of people are not comfortable with it, so in certain situations–such as when they are in a vulnerable, intimate position–they might seek assurance that their asses are a good enough size for the person who just took the opportunity to exploit that ass for his or her sexual pleasure.
It is very rare that you will find yourself in a situation in which a person with whom you are having an intimate exchange with genuinely doesn’t know how big his or her ass is and genuinely needs you to tell him or her.
Being a properly socialized adult human should give you the tools and experience to realize that you are not actually being asked for a factual description of the size of that person’s ass.
It’s all part of the interview process. This is a particularly high bar to clear–best to know early on if there is going to be a problem with it.
I do something similar–I tend to get all attentive and respectful, force the woman to voice and retain opinions which I may or may not agree with, successfully employ a broad range of humor, etc. If she’s not up to the task, she’s let go.
I knew a guy once who was convinced that a woman must have something wrong with her or she would not be with him. I don’t think he ever had a steady girlfriend in his life because he always wanted to explore exactly what it was that was wrong with them.
Dear Abby has three rules for gossip which I like and which I follow for lots more than just gossip:
She says that whatever you feel like telling must fulfill two out of three of the characteristics. Thus: “No, honey, that dress doesn’t suit you, it fits too tight” is helpful. “Your vagina is spacious” is not so much. Did I ask you for advice? If I ask you for advice on my vagina then you can say “honey try some kegels”. If I am just sitting there after sex and you think that is the right time to tell me then you probably won’t have access to it again!
There is a difference between being brutally honest and being honest. My SO and I have been together for 20 years and we are honest with each other - but we are also tactful and kind, because we love one another.
Like throwing a hotdog down a hallway, amirite guys? Yeah! hi-five
It’s been said already, but you can be honest with someone without trying to hurting them. Honesty is appreciated; brutality is not.
I think we’ve well-explored the fact that the remark was hurtful, ill-timed and the timing atrocious. I do feel terrible about saying such a thing. It’s quite out of character of me to be so lacking in tact.
But the real question that hasn’t been really addressed is why I am so open and unfiltered after intercourse? Why would I allow myself to say hurtful things under the banner of absolute honesty? Is this the real me showing itself or is this a moral failing of circumstance?
Am I trying to push the women in my life away by letting loose after sleeping with them? Do I believe I am undeserving of happiness and must subconsciously ruin my relationships?
I think this has been answered quite a few times actually.
Possibilities given are two main ones:
Your real you truly and really is a jerk/asshole. Please fix this.
You’re trying to distance yourself from women, subconsciously, by throwing mean things at them and see if they stay around or not. Please fix this.
I think most people will agree that the word love isn’t something to throw around lightly but have you thought that maybe there is a correlation between your attitude and the fact that you haven’t found anyone to love in 30 years?
Do you understand that people are not robots? Sometimes they are vulnerable, depressed and in need of support. Sometimes when you care about somebody you do for them what they need at that moment. Sometimes they might need an ego boost. That is not a bad thing. If someone is depressed about hitting 40 and you tell them they are looking old you are not being honest you are being an asshole.
Anyone else thinking of Sheldon Cooper?
Being brutal should not be part of a relationship, honesty or otherwise. Unless she is into that. In that case, the safe word is brontosaurus.